1. Field of the Invention
The present invention is directed to copper catalysts useful for hydrogenating unsaturated compositions, methods of preparing the catalysts, methods of hydrogenating unsaturated compositions and the hydrogenated products obtained therefrom.
2. Background of the Invention
The hydrogenation of unsaturated substrates is a technology widely used for obtaining products which can be used in various fields, from the food industry to the field of plastic materials and the like. Several methods are known for hydrogenation (a chemical reduction by means of adding hydrogen across a double bond), most of which use gaseous hydrogen in the presence of a suitable catalyst. The latter normally comprises a transition metal, usually a metal of group 10 of the periodic table, i.e., Ni, Pd or Pt. If these are present as impurities in the hydrogenated substrate, they can cause oxidation or toxicological problems in the case of food. Hydrogenation catalysts based on other transition metals having fewer drawbacks than those listed above are also known, but these also have a lower catalytic activity.
Hydrogenation of plant oils removes or reduces the amount of components in the oil responsible for offensive odors, poor taste and poor stability. Thus, hydrogenation provides plant oils that are useful as components for many nutritional products such as nutraceuticals and food, and for food preparation such as frying oils.
Soybean (i.e., Glycine max L. Merr.) seeds are recognized to represent one of the most important oilseed crops presently being grown in the world. Such seeds provide an excellent source of vegetable oil. While soybean oil represents an important worldwide food source, flavor and oxidative stability problems associated with its customary fatty acid composition reduces its attractiveness in some applications.
Oxidative stability relates to how easily components of an oil oxidize which creates off-flavors in the oil, and is measured by instrumental analysis such as Oil Stability Index or Accelerated Oxygen Method (AOM). The degree of oxidative stability is rated as the number of hours to reach a peroxide value of 100.
Soybean oil contains five different fatty acids (in the form of fatty acid acylglycerol esters) as its major components. These five fatty acids are: palmitic acid (C16:0) which averages about 11 percent by weight; stearic acid (C18:0) which averages about 4 percent by weight; oleic acid (C18:1) which averages about 20 percent by weight; linoleic acid (C18:2) which averages about 57 percent by weight; and linolenic acid (C18:3) which averages about 8 percent by weight of the total fatty acids. The stability problem which influences the flavor of soybean oil has been attributed to the oxidation of its fatty acids, and particularly to the oxidation of the linolenic acid (C18:3) component.
Oxidized fatty acids decompose to form volatile flavor-imparting compounds. The relative order of sensitivity to oxidation is linolenic>linoleic>oleic>saturates. Linolenic acid has been known to be the primary precursor for undesirable odor and flavor development. Since commodity soybean oil currently marketed today contains relatively high amounts of linolenic acid (7-10%) compared to other food oils such as corn oil which has about 1%, its use is constricted unless it has been hydrogenated. As a general rule the linolenic acid content should be below 1-2% in order to have the widest food application and to qualify for rigorous use environments such as for frying oils.
Soybean oil suffers from a lack of stability for frying applications due to its relatively high concentration of linolenic acid of 7 to 10%. This causes the oil to oxidize rapidly and generate off flavors and also causes early breakdown in the frying applications, resulting in premature foaming and darkening. Frying stability can be enhanced if the linolenic acid concentration can be reduced.
To address the flavor and stability problems of soybean oil due to the linolenic acid content, various processing approaches have been proposed. Such processing of the soybean vegetable oil includes: (1) minimizing the ability of the fatty acids to undergo oxidation by adding metal chelating agents, antioxidants, or packaging in the absence of oxygen; or (2) the elimination of the endogenous linolenic acid by selective hydrogenation. These approaches have not been entirely satisfactory. The additional processing is expensive, time consuming, commonly ineffective, and frequently generates undesirable by-products. While selective hydrogenation to reduce the linolenic acid content may improve oil stability somewhat, this also generates positional and geometric isomers of the unsaturated fatty acids that are not present in the natural soybean oil.
Hydrogenation can be used to improve performance attributes by lowering the amount of linolenic and linoleic acids in the oil. In this process the oil increases in saturated and trans fatty acids, both undesirable when considering health implications. In many instances, the increase in trans fatty acids is proportional to the amount of linolenic acid in the starting oil.
Due to increased knowledge of the behavior of trans fats, i.e. trans fatty acid esters, in the human body and concerns of their contributing to coronary heart disease, it is recommended that the intake of trans fats be reduced. Research has shown that diets high in saturated fats increase low density lipoproteins, which promote the deposition of cholesterol on blood vessels. More recently, dietary consumption of foods high in trans fatty acids have also been linked to a lowering of high density lipoprotein relative to low density lipoprotein and to cause an increase in inflammation. In the United States, food companies are required to label the trans content of their products above a threshold level. This has added impetus to lower the amount of trans fats in foods, particularly foods relatively high in oil, such as fried foods, including potato chips, etc. However, hydrogenation remains the primary option to convert an unstable oil to a stable oil.
Thus, polyunsaturated oils are hydrogenated to reduce the degree of unsaturation in the oil, prior to subsequent processing to obtain secondary products, such as food grade oils, additives, lubricants and the like. The content of linolenic acid (C18:3) in the oil is reduced by hydrogenation to a more saturated oil, containing increased amounts of the monoene (C18:1) and diene (C18:2).
Reduction of the double bond content in polyunsaturated oils is traditionally carried out by partial hydrogenation, catalyzed by a transition metal catalyst. Various transition metals, such as nickel, palladium and platinum have been used as hydrogenation catalysts. Catalysts vary in degree of selectivity. The selectivity referred to in this context is the ability of preferentially reducing linolenic acid before linoleic acid and oleic acid. Selectivity in this context also applies to the ability of a catalyst to reduce by hydrogenating only to form monoenes, without reducing to full saturation. Precious metal catalysts are generally the most active and also the least selective. They typically produce high amounts of saturated fatty acids for a minimal reduction of linolenic acid. Nickel catalysts are more selective and have a greater preference for reducing linolenic acid to monoene while producing less saturates. However, copper-chromium combination catalysts (i.e., copper chromite catalyst) have hitherto been found to be the most selective for production of the monoene. The hydrogenation of the polyunsaturated oils with copper chromite can produce the corresponding monoene, with little or no production of the saturated fatty acid.
Nickel catalyzed hydrogenation uses small amounts of catalyst for relatively short periods of time to reduce the linolenic acid content to the desired range, which is often 1.5%. The oil may then additionally be winterized (chilled and cold filtered) to remove any crystalline fractions. A problem with the hydrogenation processes of today is that double bonds in fatty acids can also isomerize to form trans fatty acids during hydrogenation, many of which are rare in nature. Some of these are trans fatty acids. When nickel catalysts are used, saturated and trans fatty acids are produced in high amounts relative to the desired amount of reduction of linolenic acid. This is because nickel catalysts suffer from a lack of optimum selectivity. As a result, the trans fatty acid content of oils hydrogenated with nickel catalysts can be higher than 10%.
Hydrogenation conditions to minimize trans isomer formation while reducing the oxidatively unstable species in edible oil, such as the polyunsaturated acids linolenic and linoleic acid, are currently being studied by many in the industry. Those catalysts currently being examined are generally precious metal based, and hydrogenation is carried out under extremely mild conditions, such as low temperatures. However, to date this has only resulted in a minimal decrease in trans fatty acid content in hydrogenated oils, at the cost of increased saturated fatty acid content and the use of very expensive catalysts.
Precious metal catalysts can be poisoned from various minor components in oils. As a result activity is lost over time and reaction conditions must be continually monitored and altered. These catalysts may be employed in column reactors which require emptying and recharging after the useful catalyst life has ended. The catalyst then must be returned to the catalyst company for credit and regeneration. All of this involves catalyst loss and added cost for column recycling. As precious metal catalysts lose activity and must be recovered, users of precious metal catalysts are often required to purchase a large excess of precious metal to form a “pool” or “kitty” of precious metal, so that the catalyst producer can provide fresh catalyst as needed. As a result, the use of precious metal catalysts is accompanied by a very large capital investment in precious metals.
Selective hydrogenation for producing oils for frying applications using copper chromite catalyst has been known since at least the late 1960's. Vegetable oils have been selectively hydrogenated to decrease the linolenic acid content without increasing the saturated fatty acid content constant and only minimally decreasing the linoleic acid content in soybean oil. The trans content was of no concern in those days as this was prior to the discovery of the detrimental effects of these isomers to human health. Selectively hydrogenating soybean oils produced oil with less than 2% linolenic acid and improved frying stability. However, copper chromite has low catalytic activity and requires very long reaction times. Thus reactor time is measured in hours, not in minutes, adding to increased production costs over comparable nickel catalyzed reactions.
Further, copper chromite suffers from the problem that chromium is one of the components of the catalyst, and thus any plant using this catalyst must handle the recycling and disposal of chromium in a satisfactory manner. First, the catalyst must be recovered from the oil after the hydrogenation reaction by suitable means, such as by centrifugation or filtering. Traces of catalyst remaining in the oil must be removed in a thorough manner, such as filtering through bleaching earth. This removal generates significant quantities of solid waste containing spent copper chromite catalyst and would require shipment to a land fill or to a possible reclamation facility. In addition, the finely powdered catalyst containing chromium could pose a significant health risk to workers operating the processes.
Filtered oil further requires washing with a suitable solution of chelating agent to further recover chromium. This wash water would require passage through expensive ion exchange resin columns to reduce the chromium concentration in the water prior to discharge in order to achieve allowable limits. Further, regulatory permits to allow discharge of trace levels of chromium in waste water must be obtained. In order to measure chromium released to the environment, expensive analytical monitoring equipment and trained operators would be required. Because the use of copper chromite was not attractive for the above reasons, its commercial use as a hydrogenation catalyst is obsolete.
Other copper based catalysts are known in the art. These catalysts have the advantage of being non-chromium. However, they still have the disadvantage of being no faster than copper chromite in reaction time. Furthermore, some were fabricated on a support, generally a molecular sieve, making them somewhat expensive to make. In addition, high hydrogenation temperatures were required (170 to 200° C.). To prepare these catalysts, a support material was slurried in a solution of copper (II) nitrate, and sodium carbonate was added to precipitate copper (II) carbonate onto the support. This preparation was then heated to 350° C. for two hours.
Genetic varieties of soybeans containing oil with low linolenic acid required for frying have just begun to be commercialized. The most recent variety to be commercialized has utilized a traditional genetic breeding program for its development. In general, oils produced from genetic varieties are expensive alternatives to hydrogenated oils.
There is an evident need in the fats and oils industry for an economical catalyst for soybean oil hydrogenation which selectively reduces linolenic acid without generation of significant levels of trans fatty acids or formation of saturated fatty acids.